In certain industries, such as the petroleum industry, partial stroke testing of emergency shutdown valves (ESVs) is increasingly required by regulatory bodies. However, ESVs and/or other valve assemblies that are part of “Safety Instrumented Systems” (SISs) are generally designed for on/off operation. Connections between valve stems and actuators are not tight resulting in significant lost motion. Further, ESVs are typically characterized by high seal friction and prominent stick-slip dynamics. All of these factors contribute to poor throttling control and complicate partial stoke testing.
Also, ESVs and/or other components of SISs are typically high gain devices. For example, SIS actuators are often single action pistons with a spring return. A very small change in pressure within a chamber of an actuator can cause a large movement of the piston. As a result, when coupling SISs actuators, or other SIS components, to process control devices (e.g., to perform PSTs or other tests), biases of the process control devices, such as VP (current to pressure) biases, can have a dramatic impact on the calibration of the SIS components. If a calibration of the SIS components is off by a significant amount, results from tests on the SIS components, such as partial stroke tests, will be meaningless.